As is known, direct injection internal combustion engines have a suction manifold connected to the cylinders of the engine in order to supply an air flow thereto, a fuel manifold for receiving the high-pressure fuel to supply to the said cylinders and a plurality of injectors, each of which is connected to the fuel manifold and is provided with a delivery nozzle disposed directly opposite a respective combustion chamber.
The fuel manifold receives the fuel from a high-pressure pump which is connected to the fuel storage tank, while each injector is associated with a respective cylinder and is controlled for the direct injection of fuel therein.
The supply of fuel into the cylinders is controlled by the control unit of the engine, which drives each injector by means of the generation of a respective control signal indicating the delivery time interval, i.e. the interval within which the injector must allow for the passage of fuel from the fuel manifold to the combustion chamber.
In particular, in relation to one single injection operation into one cylinder, the control unit calculates both the quantity of the fuel to be delivered and the injection advance, i.e. e.g. the time interval elapsing from the final theoretical moment of the injection to the moment at which pistons associated with the cylinder will be situated in the next upper dead centre position.
The control unit must therefore calculate and programme the delivery time interval to allow for the injection of the quantity of fuel calculated.
On the other hand, in order to be able to calculate the delivery time interval accurately, the delivery characteristics of the injector (e.g. the capacity) must be taken into account, these being highly dependent on the pressure difference present at the ends of the said injector, i.e. on the difference between the pressure present in the fuel manifold and the pressure present in the interior of the combustion chamber during the injection.
The internal pressure of the fuel manifold can generally vary between a predetermined interval including a reference pressure value (generally a pressure value of between 40 and 120 bar). The pressure in the interior of the combustion chamber, on the other hand, is highly variable, both as a function of the position of the throttle valve and as a function of the stroke of the cylinder in which the injection is supposed to take place (typically suction stroke and/or compression stroke).
Therefore, in order to be able to effectively inject the desired quantity of fuel, it is necessary to solve the problem of accurately estimating the pressure difference which will be present at the ends of the injector during the injection.
If the calculation of the delivery time interval is in fact based on the value of a pressure difference deviating significantly from the value which will actually be present during the injection, the quantity of fuel which will be supplied to the cylinder will deviate from the desired quantity. This deviation in the quantity of fuel injected is particularly harmful as it alters the air/fuel ratio with respect to the objective value, as a result of which, in addition to leading to a deterioration of the combustion, it can give rise to an increase in consumption or a loss of power, or can affect the correct operation of the catalytic converter.